Fortune Magazine's Carol Loomis, a journalist with especially strong ties to Warren Buffett, writes that a metric favored by the Omaha billionaire is now signaling it's time to buy stocks.

In today's Fortune Investor Daily on the magazine's web site, Loomis and Doris Burke point to an 85-year chart showing the the total market value of U.S. stocks as a percent of Gross National Product, a measure of economic output.

The idea is "there should be a rational relationship" between the two measures.

In a 2001 Fortune Magazine essay written by Buffett with Loomis, he says if the ratio "falls to the 70% to 80% area, buying stocks is likely to work very well for you." When it is substantially higher, "you're playing with fire." (The essay goes into extensive detail on his reasoning.)

As of late January, according to Fortune's chart, the metric had dropped to 75 percent, after hitting a peak of 190 percent in March of 2000.